1. Field of the Invention
The present invention relates to an electronic device, and in particular, to a surge absorber.
2. Description of the Related Art
Stray waves, noise, and electrostatic disturbances which may cause surges are strong obstacles to the further improvement of the most up-to-date electronic equipment. Especially, high voltage pulse waves cause erroneous operations of semiconductors in electronic equipment. These waves sometimes even damage semiconductors or the devices themselves.
Such problems can, however, be solved by the use of surge absorbers. Conventional surge absorbers produce discharge chips having an insulating microgap or discharge cores, and the discharge chips are sealed in a glass housing. For example, in a MicroAge surge absorber manufactured by Mitsubishi Materials Corporation, after a conductive thin film is grown in the ceramic core and metal cap electrodes are fitted to both edges of the core, a surface of the conductive thin film is removed by laser and a slit, or MicroAge is formed. Discharge chips (discharge cores) formed in such a manner are sealed in a glass tube. By using such a conventional chip type surge absorber, a discharge voltage can be determined based on a width of the MicroAge mentioned above (a thin groove in the form of a slit).
Further, there has been a known surge absorber which is composed of conductive films partitioned by microgrooves. However, as it is difficult to optionally select a switching voltage of such a surge absorber, the range of applications of such surge absorbers is extremely restricted. U.S. Pat. No. 4,727,350 discloses a surge absorber which comprises a cylindrical tube core covered with a conductive film having an intersecting micro groove and the exterior of which is sealed in a glass container.
The applicant of the present invention has also proposed a surge absorber in Japanese Patent Laid-Open Publication No. Hie 8-306467 which solves the conventional problems described above. According to the surge absorber, by arranging a tube core between a pair of electrodes sealed in a housing and filling a surrounding air chamber with an inactive gas, surge absorption can be achieved to a higher switching voltage then was possible in the past.
However, each of the surge absorbers mentioned above has a constitution such that discharge chips or discharge cores (tube cores) are produced in order to determine a discharge curve and the discharge chips or the discharge cores are sealed in a housing. Therefore, the constitution becomes complicated and requires a large number of production processes, and production costs cannot be reduced. Especially, when many surge absorbers must be mounted in an electronic device to protect elements or cope with the fluctuation of power-supply voltage, a number of surge absorbers must be used which directly leads to a problem that the equipment cost of the complete device rises.
Further, according to surge absorbers proposed heretofore, a discharge current flows via a tube core, whereby it is difficult to cope with a high switching voltage of ten thousand volts and to completely absorb a surge of large energy at the time of surge absorption. This causes a problem that, due to the residual voltage, a dynamic current (a current which flows into electronic equipment to be protected due to the presence of a residual voltage) arises in a circuit. Further, in the conventional devices, there is a problem that a switching voltage varies depending on specifications of the tube core.
The present invention is made in consideration of the conventional problems described above and the object is to provide an absorber which can easily be produced in bulk due to its remarkably simple structure and is applicable to a wide range of surge voltage and a surge withstand capacity.
The present invention is directed to providing a surge absorber which is capable of performing surge absorption over a wide range of operating voltages, absorbing instantaneously large energy by remarkably reducing resistance at the time of surge absorption, and eliminating with certainty any residual voltage remaining after conventional surge absorption and any dynamic current which arises resulting from the residual voltage. Further, the surge absorber is an improvement which is capable of fine adjustment of the discharge voltage, discharge speed, and surge withstand capacity (surge current) by allowing optional design of each part of the surge absorber.
In order to achieve the object described above, the present invention is characterized in that a pair of discharge electrodes having lead terminals are arranged face to face at a prescribed distance in a housing, the housing is melted while the prescribed distance is maintained, and both edges of the housing are welded to the electrodes or the lead terminals.
Therefore, according to the present invention, the pair of discharge electrodes are accurately maintained in an arrangement such that they face each other at a prescribed distance in the housing, and, with this arrangement maintained, the electrodes or the lead terminals are sealed by heat welding the housing. Thus, it is possible to optionally select the distance between these two discharge electrodes and facilitate precise adjustment of the distance.
Heretofore, a gas tube arrested manufactured by Ishizuka Electronics Corporation has been known as a typical surge absorber without chips. This conventional device has constitution such that electrodes are arranged facing each other at a prescribed distance maintained by insulating materials, such as glass. However, in the gas tube arrested, a distance between the opposed electrodes is determined according to the length of an insulating tube, and the insulating tube and the electrodes are welded. Such constitution requires the preparation of a great variety of insulating tubes of varied lengths in order to obtain various kinds of opposed electrodes, namely, opposed electrodes separated by varied distances, and it is substantially impossible to obtain a surge absorber without chips which is applicable to a wide range of discharge voltage and a surge withstand capacity. Further, as in insulating tubes made of the glass, since the distance between electrodes is determined according to the length of an insulating tube, the distance between the electrodes fluctuates when the insulating tube and the electrodes are welded by heating. Under the circumstances, heat welding cannot be used, and the insulating tube and the electrodes must be welded at their opposed surfaces. Thus, due to flux or the like which arises at the time of welding, severe contamination occurs in an air chamber, thereby leading to remarkable deterioration of discharge curves.
In contrast with the conventional device described above, the present invention has an advantage that various types of accurate distances between electrodes can be easily obtained because the housing is welded by heating under the condition that a distance between a pair of opposed electrodes is accurately maintained independent of the housing.
Further, the present invention is characterized in that it has a housing, a pair of discharge electrodes which are arranged facing to each other in the housing and connected to leads or terminals, respectively, and an air chamber formed between the pair of discharge electrodes, and in that clean and dry air, a mixed gas of the clean and dry air with an inactive gas, or a mixed gas of the clean and dry air with an nitrogen gas is sealed in the chamber.
Therefore, according to the present invention, utilizing a prescribed air gap, the pair of discharge electrodes are sealed in the housing, and clean and dry air, or a mixed gas of the clean and dry air with an inactive gas or a nitrogen gas is sealed in the air chamber. Thus, with a remarkably simple structure, it is possible to cope with surges our a wide range of switching voltage. Further, since gas resistance in the air chamber is very low at the time of insulation discharge, the operating resistance when dielectric breakdown of gas arises resulting from a surge voltage is very low. Thus, a surge of high switching voltage can be instantaneously absorbed, and residual voltage which has arisen heretofore can be effectively prevented. A surge absorber in which an air gap is merely provided between electrodes is well known as the gas tube arrested described above. However, according to the present invention, by sealing sufficiently clean and dry air in the air chamber and performing in a stable manner a dielectric breakdown of the inside the air chamber arranged between the opposed electrodes, it is possible to securely provide a very useful surge absorption path.
A further aspect of the present invention is characterized in that clean and dry air with humidity of five percent or less and cleanliness of 99.99 percent (0.5 xcexcmop), which is higher than cleanliness to be obtained through filtration of normal air, is sealed in an air chamber.
In yet another aspect of the present invention, at least one of the pair of discharge electrodes forms a flat discharge surface which is in contact with an air gap.
The surge absorber without chips according to the present invention may be a container made airtight with glass or plastic.
Air to be sealed in the air chamber according to the present invention is not normal air, but clean and dry air, as described above. Its cleanliness is 99.99 percent (0.5 xcexcmop) which is higher than cleanliness to be obtained through filtration of normal air. With regard to dryness, humidity is five percent or less, preferably three percent or less. Further, as the occasion demands, for example, when required to adjust a surge switching voltage, air to be sealed in the air chamber can be mixed with other inactive gas or the like. It is preferable to use argon or neon as a mixed inactive gas. It is also preferable to use nitrogen instead of such a mixed inactive gas.
The surge absorber without chips described above can be widely used in very complicated electronic circuits which are important components for resetting a high speed computer having a mass storage memory. Therefore, it is possible to effectively exclude the influence of surge waves resulting from frequent ON/OFF operations of a computer display or other electronic equipment.
Further, the surge absorber without chips according to the present invention can also be used in devices to be connected to telephone lines, such as a telephone set, a radio, a facsimile, a modem, and a program controlled telephone exchanger; devices to be connected to antennas or signal conductors, such as an amplifier, a tape recorder, a vehicle radio, a radio transceiver, and a sensor signal conductor; devices required for electrostatic prevention, such as a display and a monitor; domestic appliances; and computer controlled electronic equipment. The surge absorber without chips also functions as an over voltage prevention device. In other words, it is an electronic device effective for counteracting the hazardous influence of static electricity.